The loads of N and P in Chinese rivers are at alarming levels. This is mainly because of agricultural transitions increasing the direct discharge of animal manure in rivers (Strokal et al. 2016b). Thus the potential for coastal eutrophication is high. We show that by 2000 dissolved N and P export by rivers to the Bohai Gulf had increased by a factor of 6-7, to the Yellow Sea by a factor of 2-5 and to the South China Sea by a factor of 2-4 relative to 1970 (ranges are for nutrient forms, Figure 6.3, Text S6.2). As a result, in 2000 the Bohai Gulf received 83-212 kton of N and 3-41 kton of P, the Yellow Sea received 561-594 kton of N and 37-112 kton of P, and the South China Sea received 182- 375 kton of N and 21-33 kton of P (the ranges are for dissolved inorganic and dissolved organic forms) (Figure 6.3, Text S6.2). ICEP values are positive for 2000, indicating the potential for coastal eutrophication (Figure 6.6). Strokal et al. (2016a) show details in past trends in nutrient export by rivers to Chinese seas and main causes of the increasing trends. We summarized these trends in the Supplementary Materials (Text S6.2).
In 2050, the projected coastal water pollution is low, but only in optimistic scenarios (OPT-1 and OPT-2, Figures 6.3-6.5). The GO scenario, which is the worst case for the environment, projects a high potential for coastal eutrophication (ICEP>0, Figure 6.6). This is because rivers are projected to export roughly 10-90% more nutrients in 2050 than in 2000 (range for rivers, Figure 6.3). An exception is DIN export by the Hai River to the Bohai Gulf for which we calculate a decrease between 2000 and 2050 because of river damming. The large Yellow, Yangtze and Pearl rivers are projected to transport relatively large amounts of dissolved N and P to the Bohai Gulf, Yellow Sea and South China Sea, respectively (Figures 6.3-6.5, Figure S6.14). Over half of the nutrients from these rivers may result from middle- and downstream activities (Figure S6.15). Manure point sources will remain a major polluter of rivers (except for DIN) because animal production will continue to industrialize without proper manure management (Figures 6.4, 6.5, and S6.14). Synthetic fertilizers will further contribute to DIN inputs in the Yangtze and Pearl. The share of urban human waste in river pollution is projected to increase (Figure S6.14) especially for DIP in rivers of downstream sub-basins (Figure 6.5, Figure S6.15). This is driven by urbanization (Figure S6.9).
147 Implementing current policy plans in agriculture (the CP scenario) may not be effective enough to reduce future nutrient pollution of most Chinese rivers (Figures 6.3-6.5). Thus the high potential for coastal eutrophication may remain in 2050 (ICEP>0, Figure 6.6). In the CP scenario nutrient export by most rivers is projected to increase between 2000 and 2050, but not as fast as in GO. In particular P export is lower (Figures 6.3-6.5, S6.14). The differences between CP and GO are associated with manure recycling on land and use of synthetic fertilizers in 2050. CP incorporates the recent policy (MOA 2015) in which manure recycling (60%) is somewhat higher than in GO (around 50%, Table S6.1). Thus in CP the contribution of manure point sources to river pollution is somewhat lower than in GO. This explains why in CP nutrient export by most rivers is only slightly lower than in GO. In contrast, DOP export by Hai and Huai is decreasing between 2000 and 2050 in CP. Thus recycling 60% of the manure may be enough to reduce DOP export by these rivers. Synthetic fertilizers remain important sources of DIN in the Yangtze and Pearl rivers, as in GO. This is because the use of N synthetic fertilizers in CP is comparable to that in GO (Figure S6.8). This is different for P synthetic fertilizers. CP projects much lower use of P fertilizers than GO. This explains why effects of this policy on DIP export by the Yangtze and Pearl Rivers are more visible than on other nutrient forms (Figures 6.3-6.5, S6.14). In other rivers the majority of DOP and DIP is from point sources (Figure 6.5) and thus effects of reduced P fertilizers on land is negligible.
In scenarios OPT-1 and OPT-2, river export of nutrients is projected to reduce to levels with low potentials for coastal eutrophication (ICEP<0, Figures 6.3-6.6). For 2050 these optimistic scenarios project river export of most nutrients to be at around levels of 1970 (Figures 6.3, S6.14). Only DIN export by the Yangtze and Pearl is projected to be at around levels of 2000 (Figures 6.3-6.5). Low river export of nutrients in OPT-1 is largely associated with improved manure management (Tables 6.1, S6.1 and S6.2). In particular, maximum recycling of the manure on land avoids direct discharge of manure to Chinese rivers.
As a result, the nutrient pollution of Chinese seas reduced considerably (Figures 6.3-6.5, S6.14). Furthermore, the use of N and P synthetic fertilizers is projected much lower in OPT-1 than in GO and CP (Figures S6.6-S6.8, Tables 6.1, S6.1 and S6.2). This further reduces DIN export by the Yangtze and Pearl rivers in 2050 for which synthetic fertilizers are important sources (Figure S6.14). In OPT-1 human waste becomes the dominant source of nutrients in many rivers (Figures 6.4 and 6.5). Leaching of organic matter is an important source of DON in OPT-1 and for biological N fixation and atmospheric N deposition of DIN in the Yangtze and Pearl.
6
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6.3 Reducing future coastal water pollution in China
The loads of N and P in Chinese rivers are at alarming levels. This is mainly because of agricultural transitions increasing the direct discharge of animal manure in rivers (Strokal et al. 2016b). Thus the potential for coastal eutrophication is high. We show that by 2000 dissolved N and P export by rivers to the Bohai Gulf had increased by a factor of 6-7, to the Yellow Sea by a factor of 2-5 and to the South China Sea by a factor of 2-4 relative to 1970 (ranges are for nutrient forms, Figure 6.3, Text S6.2). As a result, in 2000 the Bohai Gulf received 83-212 kton of N and 3-41 kton of P, the Yellow Sea received 561-594 kton of N and 37-112 kton of P, and the South China Sea received 182- 375 kton of N and 21-33 kton of P (the ranges are for dissolved inorganic and dissolved organic forms) (Figure 6.3, Text S6.2). ICEP values are positive for 2000, indicating the potential for coastal eutrophication (Figure 6.6). Strokal et al. (2016a) show details in past trends in nutrient export by rivers to Chinese seas and main causes of the increasing trends. We summarized these trends in the Supplementary Materials (Text S6.2).
In 2050, the projected coastal water pollution is low, but only in optimistic scenarios (OPT-1 and OPT-2, Figures 6.3-6.5). The GO scenario, which is the worst case for the environment, projects a high potential for coastal eutrophication (ICEP>0, Figure 6.6). This is because rivers are projected to export roughly 10-90% more nutrients in 2050 than in 2000 (range for rivers, Figure 6.3). An exception is DIN export by the Hai River to the Bohai Gulf for which we calculate a decrease between 2000 and 2050 because of river damming. The large Yellow, Yangtze and Pearl rivers are projected to transport relatively large amounts of dissolved N and P to the Bohai Gulf, Yellow Sea and South China Sea, respectively (Figures 6.3-6.5, Figure S6.14). Over half of the nutrients from these rivers may result from middle- and downstream activities (Figure S6.15). Manure point sources will remain a major polluter of rivers (except for DIN) because animal production will continue to industrialize without proper manure management (Figures 6.4, 6.5, and S6.14). Synthetic fertilizers will further contribute to DIN inputs in the Yangtze and Pearl. The share of urban human waste in river pollution is projected to increase (Figure S6.14) especially for DIP in rivers of downstream sub-basins (Figure 6.5, Figure S6.15). This is driven by urbanization (Figure S6.9).
147 Implementing current policy plans in agriculture (the CP scenario) may not be effective enough to reduce future nutrient pollution of most Chinese rivers (Figures 6.3-6.5). Thus the high potential for coastal eutrophication may remain in 2050 (ICEP>0, Figure 6.6). In the CP scenario nutrient export by most rivers is projected to increase between 2000 and 2050, but not as fast as in GO. In particular P export is lower (Figures 6.3-6.5, S6.14). The differences between CP and GO are associated with manure recycling on land and use of synthetic fertilizers in 2050. CP incorporates the recent policy (MOA 2015) in which manure recycling (60%) is somewhat higher than in GO (around 50%, Table S6.1). Thus in CP the contribution of manure point sources to river pollution is somewhat lower than in GO. This explains why in CP nutrient export by most rivers is only slightly lower than in GO. In contrast, DOP export by Hai and Huai is decreasing between 2000 and 2050 in CP. Thus recycling 60% of the manure may be enough to reduce DOP export by these rivers. Synthetic fertilizers remain important sources of DIN in the Yangtze and Pearl rivers, as in GO. This is because the use of N synthetic fertilizers in CP is comparable to that in GO (Figure S6.8). This is different for P synthetic fertilizers. CP projects much lower use of P fertilizers than GO. This explains why effects of this policy on DIP export by the Yangtze and Pearl Rivers are more visible than on other nutrient forms (Figures 6.3-6.5, S6.14). In other rivers the majority of DOP and DIP is from point sources (Figure 6.5) and thus effects of reduced P fertilizers on land is negligible.
In scenarios OPT-1 and OPT-2, river export of nutrients is projected to reduce to levels with low potentials for coastal eutrophication (ICEP<0, Figures 6.3-6.6). For 2050 these optimistic scenarios project river export of most nutrients to be at around levels of 1970 (Figures 6.3, S6.14). Only DIN export by the Yangtze and Pearl is projected to be at around levels of 2000 (Figures 6.3-6.5). Low river export of nutrients in OPT-1 is largely associated with improved manure management (Tables 6.1, S6.1 and S6.2). In particular, maximum recycling of the manure on land avoids direct discharge of manure to Chinese rivers.
As a result, the nutrient pollution of Chinese seas reduced considerably (Figures 6.3-6.5, S6.14). Furthermore, the use of N and P synthetic fertilizers is projected much lower in OPT-1 than in GO and CP (Figures S6.6-S6.8, Tables 6.1, S6.1 and S6.2). This further reduces DIN export by the Yangtze and Pearl rivers in 2050 for which synthetic fertilizers are important sources (Figure S6.14). In OPT-1 human waste becomes the dominant source of nutrients in many rivers (Figures 6.4 and 6.5). Leaching of organic matter is an important source of DON in OPT-1 and for biological N fixation and atmospheric N deposition of DIN in the Yangtze and Pearl.
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In OPT-2, low river export of nutrients in 2050 is a result of assumptions on the efficient use of nutrients both in agriculture (as in OPT-1) and sewage (Tables 6.1, S6.1 and S6.2). In general, this OPT-2 scenario projects lower river export of most nutrients than OPT-1 (Figures 6.3-6.5). This is because OPT-2 assumes full access to improved sanitation with efficient nutrient treatment in centralized (for urban people) and decentralized (for rural people) systems as well as banned P-based detergents. The effect of this improved sanitation is larger on P export by rivers from urbanized sub-basins. In 2050, urbanization may especially be higher in downstream sub-basins (e.g., Hai, Huai, downstream sub-basins of the Yellow, Yangtze and Pearl rivers), and in some middlestream sub-basins of the Yangtze and Yellow rivers (see Figure 6.1 for location of sub-basins, Figures S6.9-S6.11 for urbanization). In OPT-2 the share of other sources in nutrient export (e.g., leaching of organic matter, P weathering, biological N fixation and atmospheric N deposition) may increase in 2050 especially for rivers of the Yellow Sea and South China Sea (Figures 6.4, 6.5, and S6.14).
149